1. Field of the Invention
The invention relates in general to a coupling circuit and associated method, and more particularly, to coupling circuit and associated method for an integrated circuit.
2. Description of the Related Art
In a signal transmission system, a transmission signal from a transmission end may include an alternating-current (AC) component and a direct-current (DC) component. As the DC component may get lost or drift during the transmission, a reception end cannot determine a DC reference level based on the DC component in the reception signal. Therefore, before the reception end of the signal transmission system processes the reception signal, the reception signal is AC coupled by an external capacitor to remove the DC component from the reception signal. The reception end then reconstructs the DC component in the reception signal according to a predetermined level for subsequent processes, or directly processes the reception signal without reconstructing the DC component in the reception signal.
The removal and reconstruction of the DC component is applied to various transmission systems. An image signal transmission system is taken as an example for further illustrations. FIG. 1 shows a schematic diagram of a reception end of an image signal transmission system. Referring to FIG. 1, in the image signal transmission system, a synchronization signal is added to an image signal (to be jointly referred to as a composite signal) and transmitted to an image signal reception integrated circuit 100. Before entering the integrated circuit 100, the composite signal is AC coupled with an external capacitor 110 to remove a DC component from the composite signal. A synchronization signal detection circuit 120 in the signal reception integrated circuit 100 then reconstructs a DC level of the composite signal and detects the synchronization signal in the component signal to obtain synchronization information, which is later utilized in subsequent image processes. In general, the synchronization signal detection circuit 120 includes a clamping circuit 130 for limiting the DC level of the composite signal within a range according to a predetermined level, and a comparison circuit 140 for detecting the synchronization signal in the composite signal to obtain the foregoing synchronization information for subsequent image processes. For example, when the synchronization signal is added to a G signal representing the green color in an RGB signal, the G signal is usually referred to as a sync-on-green (SOG) signal; when the synchronization signal is added to a Y signal representing luminance in a YPbPr signal, the Y signal is usually referred to as a sync-on-luminance (SOY) signal. For illustration purposes, an image signal containing a synchronization signal is referred to as a composite signal throughout the specification.
Referring to FIG. 1, the image signal reception integrated circuit 100 includes an analog front-end circuit 150. In addition to the foregoing synchronization signal detection circuit 120, the analog front-end circuit 150 further receives the composite signal via another external capacitor 160 to obtain image information. In current techniques, the integrated circuit 100 receives the composite signal via the two external capacitors 110 and 160 by using different pins, and provides the synchronization information to the synchronization signal detection circuit 120 and the image information to the analog front-end circuit 150. In other words, the synchronization detection circuit 120 obtains the synchronization information by receiving the composite signal via an independent pin. However, if the above independent pin is eliminated, i.e., if the synchronization signal detection circuit 120 directly and internally obtains the synchronization information in the composite signal from the integrated circuit 100, the image signal transmission system may become more cost-effective.